Understanding the network.pdf - Back to Home

• Support for full-duplex operation—When multiport bridges were introduced
for 10Mbps Ethernet, one of the derivative features was full-duplex
operation for 10Base-T devices. This enhancement, however, was added out
of the 802.3 specification. The 802.3u standard defines a mechanism for
full-duplex operation, recognizing its valuable potential in terms of actual
performance and possible use in supporting larger 100Mbps segment
topologies.
• Exclusive use of an active star topology—Only a point-to-point "active hub"
(end-station to repeater) transmission media model is supported. There is
no PHY specification for "passive bus" transmission media such as 10Base-5.
Passive bus transmission media cannot easily support the simultaneous
dual-speed operation and transmission and receive functionality required for
full-duplex operation.
The 802.3u standard defines two UTP implementations: 100Base-T4, which
operates over four pairs of Category 3 UTP, and 100Base-TX, which utilizes two
pairs of Category 5 UTP. The 100Base-FX standard operates over a single pair of
62.5/125 or 50/100 micron multimode fiber. 100Base-T2, a third UTP
implementation of 100Mbps operation using only two pairs of Category 3 UTP, was
added a year or so after the 100Base-T standard was finalized. No commercial
products have ever shipped using this standard.
10Mbps Ethernet employed Manchester encoding for both the MAC-to-PDM
communication between the end-station controller and its MAU and for
end-station-to-end-station transmissions across the network medium. As
mentioned earlier, this approach is not suitable for high-speed data transmission.
Therefore, a new set of PHY interfaces was developed based on the FDDI Physical
Media Dependent (PMD) sublayer mechanisms. There are six 100Mbps PHY
interfaces or sublayers (listed from the MAC down).
The Media Independent Interface was developed to provide interpretability between
the MAC layer and the new PHY implementations. This interface defined a Physical
Coding Sublayer (PCS), which performs data encoding, data transmission and
receive functions, and CSMA/CD duties. The PCS in essence performs the same
duties as the 10Mbps PLS interface. The Physical Medium Attachment (PMA)
interface handles the mapping of the PCS to and from the transmission medium.
The PMD interface defines the connection interface and the usage designations for
the transmission medium elements—for example, which wire pairs are used for
what function. The Auto-Negotiation (AutoNeg) interface exists between the NIC
and the repeater as a management interface to determine the link's operating rate.
Auto-negotiation is not supported on all 100Mbps PHY implementations. It is
accomplished by using the normal link pulse and fast link pulse tests employed by
repeaters to check link integrity when an end-station comes online and establishes
its connection with the repeater. The repeater uses the link pulses to determine the
optimal operating rate.